Our cat was around 6 months old when we started potty training. It takes some time for the cat to get used to the new location. You have to take each step slowly and not rush things. If you rush you get misses. You basically move the litter box onto the toilet then make a bigger and bigger hole it in until the cat is standing on the seat. I never looked for forums about this.
For an older cat you may need to take things a little slower, but I'd think as long as the cat could jump up on the toilet you should be able to train it.